An electrophotographic two-component developer consists of a toner and a carrier, in which the carrier is used for the purpose of providing a proper polarity and a proper amount of triboelectric charge to the toner.
As the carrier, there is used a resin-coated carrier comprising a resin coat layer formed on the surface of each core material particle.
As for the resin-coated carrier there are conventionally known techniques shown below:
(1) The technique disclosed in Japanese Patent Examined Publication No. 48782/1982, in which a carbon fluoride-added fluororesin is used as a coating material for forming a resin coat layer on the carrier.
(2) The technique disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP O.P.I.) No. 48050/1985, in which carbon fluoride is added as conductive particles to the resin coat layer of a carrier.
The incorporation of carbon fluoride into the resin coat layer as disclosed in the above techniques (1) and (2) enables to lower the surface energy of the carrier, so that a carrier causing less toner permanent welding of toner particles to the carrier surface can be obtained.
However, in a wet process which uses a coating liquid as a coating means, the aggregative power of carbon fluoride particles is so strong that it is considerably difficult for the particles to uniformly disperse in a state of primary particles in a coating liquid. Therefore the carbon fluoride particles are present in a secondary aggregate state in the coating liquid to thus have a very poor dispersion stability.
The poor dispersion stability of carbon fluoride particles in the coating liquid makes it difficult to handle the coating liquid and causes the carbon fluoride to disperse unevenly in the formed resin coat layer, and further worsen the adhesion of the coating resin to the carbon fluoride.
When such a carrier is used to form a number of image copies, since the carbon fluoride is liable to split from the resin coat layer, the characteristics of the carrier largely change with time to cause the carrier's durability to be insufficient.
The triboelectric charging with the toner depends largely upon the characteristics of the outermost surface of the resin-coated carrier, but even on the outermost surface of the carrier itself the coating resin and carbon fluoride are unevenly dispersed, so that the difference in the chargeability between the coating resin and the carbon fluoride makes the toner unable to be uniformly triboelectrically charged to result in charging trouble of the developer to allow an increased amount of counter polarity-having toner particles to be present to cause image defects such as a background fog and solid image density drop of a copied image.